Imagine this; your book is about to be turned into a movie.
Which actor/actress do you think is the most ideal person to play your character?
Here's a few points to remember when selecting the actor:
* If your MC is not a human character, then perhaps you could suggest the actor/actress who is best suited to provide a voiceover.
* Post a link of the actor/actress so that everyone can see (I'm not sure about the rules regarding uploading images, so let's not break the rules and stay on the safe side).
* It doesn't necessarily have to be a Hollywood actor. The person could be Japanese, Norwegian, British, Korean, Indian or from anywhere else. They just have to be listed as an actor in a movie.
* If you can't find the right person, then think of the 'younger version' of an actor. For example, let's say your character should be played by Clint Eastwood, you could post a link of a younger version of Mr. Eastwood or just let everyone know that a younger version of said actor could play the role.
Also, drop in a link to your book so that we can check out why you think Ryan Reynolds or Naomi Watts should play your character 
For my book, I would say that a younger version of Jesse Williams would be perfect.